Formatting in Haskell

By Chris Done

This post is about the formatting package.

What’s wrong with printf?

The Text.Printf module is problematic simply because it’s not type-safe:

λ> import Text.Printf
λ> printf "" 2
*** Exception: printf: formatting string ended prematurely
λ> printf "%s" 2
*** Exception: printf: bad formatting char 's'

And it’s not extensible in the argument type. The PrintfType class does not export its methods.

And it’s not extensible in the formatting. You can’t add a “%K” syntax to it to print a value in Kelvins, for example.

And it’s implicit. You can’t just use your normal API searching facilities to search how to print a Day.

Holy Moly!

A while ago I was inspired by the HoleyMonoid package to use that mechanism to make a general replacement for printf.

It’s a continuation-based way of building up monoidal functions by composition with the ability to insert constants in-between. Example:

let holey = now "x = "
          . later show
          . now ", y = "
          . later show

> run holey 3 5
"x = 3, y = 5"

The now function inserts a monoidal value directly into the composition. So

run (now x . now y)

is equivalent to

x <> y


run (later show . now x . later show . now y)

is equivalent to

\a b -> show a <> x <> show b <> y

The Formatting package

The package is available on Hackage as formatting.

Comparison with other formatters


format ("Person's name is " % text %  ", age is " % hex) "Dave" 54

or with short-names:

format ("Person's name is " % t % ", age is " % x) "Dave" 54

Similar to C’s printf:

printf("Person's name is %s, age is %x","Dave",54);

and Common Lisp’s FORMAT:

(format nil "Person's name is ~a, age is ~x" "Dave" 54)

The Holey type

newtype HoleyT r a m = Holey { runHM :: (m -> r) -> a }

type Holey m r a = HoleyT r a m

This is my version of the HoleyMonoid. To make this into a useful package I changed a few things.

The Category instance implied a name conflict burden with (.), so I changed that to (%):

(%) :: Monoid n => Holey n b c -> Holey n b1 b -> Holey n b1 c

Rather than have the name-conflicting map function, I flipped the type arguments of the type and made it an instance of Functor.


There is an array of top-level printing functions for various output types:

-- | Run the formatter and return a lazy 'Text' value.
format :: Holey Builder Text a -> a

-- | Run the formatter and return a strict 'S.Text' value.
sformat :: Holey Builder S.Text a -> a

-- | Run the formatter and return a 'Builder' value.
bprint :: Holey Builder Builder a -> a

-- | Run the formatter and print out the text to stdout.
fprint :: Holey Builder (IO ()) a -> a

-- | Run the formatter and put the output onto the given 'Handle'.
hprint :: Handle -> Holey Builder (IO ()) a -> a

All the combinators work on a lazy text Builder which has good appending complexity and can output to a handle in chunks.

The Format type

There is a short-hand type for any formatter:

type Format a = forall r. Holey Builder r (a -> r)

All formatters are written in terms of now or later.


There is a standard set of formatters in Formatting.Formatters, for example:

text :: Format Text
int :: Integral a => Format a
sci :: Format Scientific
hex :: Integral a => Format a

Finally, there is a general build function that will build anything that is an instance of the Build class from the text-format package:

build :: Buildable a => Format a

For which there are a bunch of instances. See the README for a full set of examples.

Composing formatters

%. is like % but feeds one formatter into another:

λ> format (left 2 '0' %. hex) 10


You can include things verbatim in the formatter:

> format (now "This is printed now.")
"This is printed now."

Although with OverloadedStrings you can just use string literals:

> format "This is printed now."
"This is printed now."

You can handle things later which makes the formatter accept arguments:

> format (later (const "This is printed later.")) ()
"This is printed later."

The type of the function passed to later should return an instance of Monoid.

later :: (a -> m) -> Holey m r (a -> r)

The function you format with (format, bprint, etc.) will determine the monoid of choice. In the case of this library, the top-level formating functions expect you to build a text Builder:

format :: Holey Builder Text a -> a

Because builders are efficient generators.

So in this case we will be expected to produce Builders from arguments:

format . later :: (a -> Builder) -> a -> Text

To do that for common types you can just re-use the formatting library and use bprint:

 > :t bprint
bprint :: Holey Builder Builder a -> a
> :t bprint int 23
bprint int 23 :: Builder

Coming back to later, we can now use it to build our own printer combinators:

> let mint = later (maybe "" (bprint int))
> :t mint
mint :: Holey Builder r (Maybe Integer -> r)

Now mint is a formatter to show Maybe Integer:

> format mint (readMaybe "23")
> format mint (readMaybe "foo")

Although a better, more general combinator might be:

> let mfmt x f = later (maybe x (bprint f))

Now you can use it to maybe format things:

> format (mfmt "Nope!" int) (readMaybe "foo")


I’ve been using formatting in a bunch of projects since writing it. Happily, its API has been stable since releasing with some additions.

It has the same advantages as Parsec. It’s a combinator-based mini-language with all the same benefits.